The electronics industry worldwide is adopting halogen-free products in line with the trends of environmental protection and green regulations. Countries and large electronics companies have set a timetable for large scale production of halogen-free electronic products. Under the Restriction of Hazardous Substances (RoHS) Directives implemented by the European Union, electronic products and components are not allowed to contain lead (Pb), cadmium (Cd), mercury (Hg), hexavalent chromium (Hex-Cr), polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE). Because printed circuit board (PCB) is the foundation of electronic and electrical products, the halogen-free requirement first targets PCB products. Many international organizations have set out rigorous requirements for halogen content in PCB. The International Electrotechnical Commission (IEC) 61249-2-21 stipulates that maximum limit of chlorine and bromine in products is less than 900 ppm respectively, and the maximum limit of the overall halogen is less than 1500 ppm. The Japan Electronic Packaging and Circuits Association (JPCA) also requires the contents of chlorine and bromine to be under 900 ppm. The Green Peace Association is also pushing hard the green initiatives, demanding all manufacturers to remove poly(vinyl chloride) and brominated flame retardant completely from their electronic products to meet the definition of lead-free and halogen-free green electronics. Thus companies are now focusing on the development of halogen-free materials.
New generations of electronic products stress light, thin, short and small, and suitable for high-frequency transmission. Thus the PCB layout is gearing towards high density with more rigorous requirements for the choice of PCB material. High-frequency electronic components are connected to the PCB. To maintain the transmission speed and signal integrity, the PCB laminate material should have lower dielectric constant and lower dissipation factor. Also to make sure electronic components will function normally under high temperature and high humidity environment, the PCB must offer the features of heat resistance, flame retardancy and low moisture absorption. Having good adhesion strength, heat resistance and formability, epoxy resin has been used extensively in copper clad laminate or as sealing material in electronic and electromechanical components. From the perspective of fire safety, materials are required to possess flame retardancy. Usually non-flame retardant epoxy resin is externally added with flame retardant to achieve the flame retardant effect. For example, halogen, particularly bromine, is introduced into the epoxy resin to enhance the reactivity of epoxy group and endow it with flame retardant property. In addition, halide could decompose after long-term use under high temperature and cause the corrosion of minute wiring. The disposed electronic components will also produce halide and other hazardous compounds after burning, which is unfriendly to the environment. To replace the halogenated flame retardants, there have been studies on using phosphorus compound as flame retardant by, for example, adding phosphate ester or red phosphorus to the epoxy resin composition. However phosphate ester would undergo hydrolysis to separate acid, hence affecting the migration resistance of epoxy resin. Red phosphorus might have high flame retardancy, but it is considered a hazardous substance in fire safety law, which could produce minute amount of phosphine gas in high temperature and moist environment.
The conventional circuit board technology for making copper clad laminate uses an epoxy resin and a hardener as the raw materials of thermosetting resin composition, where, reinforcement material (e.g. fiberglass cloth) and the thermosetting resin composition are heated and adhered together to form a prepreg. The prepreg, sandwiched between two copper foil sheets, is pressed into a copper clad laminate. Prior technology commonly uses epoxy resin and hydroxy (—OH) containing phenol novolac hardener as the raw materials of thermosetting resin composition. The reaction of phenol novolac and epoxy resin would open the epoxy ring to form another hydroxyl group. Hydroxyl group would increase the dielectric constant and dielectric loss (dissipation factor) of the composition and tends to bind to water molecules to increase the hygroscopic property of the composition.
A conventional thermosetting resin composition comprising cyanate ester based resin, dicyclopentadiene based epoxy resin, fused silica, and thermoplastic resin. This thermosetting resin composition has low dielectric constant and low dissipation factor. But it uses halogen (e.g. bromine) containing flame retardant, such as tetrabromocyclohexane, hexabromocyclodecane and 2,4,6-tris(tribromophenoxy)-1,3,5-triazine in the manufacturing process. The use, recycle, or even disposal of such products that contain brominated flame retardant tend to pose a challenge to the environment. To improve the heat resistance, low dissipation factor, low hygroscopicity, high crosslinking density, high glass transition temperature, high adhesion strength, proper thermal expansion of copper clad laminate, the choice of epoxy resin, curing agent and reinforcement materials become key considerations.
In terms of electrical properties, major consideration should be given to the dielectric constant and dielectric loss (also called “dissipation factor”) of the material. In general, the signal transmission speed of the laminate is inversely proportional to the square root of its dielectric constant. That is why the smaller the dielectric constant of the laminate material is better. On the other hand, lower dissipation factor means less loss in signal transmission. Thus materials with low dissipation factor provide better transmission quality.
How to develop materials with low dielectric constant and low dissipation factor and apply them to the manufacturing of high-frequency PCB is a pressing problem that PCB material suppliers need to address at the present time.
In light of the drawbacks of prior art, the inventor, based on his many years of experience in the industry, develops a halogen-free resin composition that meet the objectives of low dielectric constant, high heat resistance and high flame retardancy.